Ive dealt with nightmare demons in the past. Their not all that powerful, and are even weaker than novices. Their lifespan before they fade is about as long as the person pays attention to them in their dreams. That's why people of the wiccan world generally dont deal with them. But the easiest way to get rid of them is cleanse the crown and third eye chakra.
As for demons who feed off of nightmares, they leave marks on the bodies they siphon the nightmares, fear, and dream energy from. Those are harder to get rid of, since their connection only becomes active when the victim is sleeping.